What do I do when I know too much about a police case?

So, I have a few police detective friends... One called me and our other female friend to meet up. Come to find out our other friends ex had been involved in a domestic despute/ suicide. She had dated the man for 4 years, was engaged to be married, but he cheated. Fast forward 3 years and he meets another woman and moved in with her right away and their relationship is constant fighting, drinking, drugs, breakup/makeup the whole time. My friend hadn't seen or heard from him in 3 years. So the detective begins to tell her that apparently both physically and mentally abused eachother, both seem to of cheated on eachother many times, and we're both alcoholics and druggies. They got in a fight and he told her to pack her stuff and go. So she went up stairs and instead of packing decided to make him sorry by shooting herself in the chest. The detectives believed it was to make him feel guilty, take her back and not to actually die due to the location of the wound. He heard the gunshot, ran upstairs and found her and called the cops and tried to prevent the bleeding but she had passed before the cops arrived.

I had never even met him so the officer figured no biggie if I knew. My friend obviously took it very badly.

The next day however I got a call from my cousin, the woman who shot herself was her childhood best friend! My cousin and the woman's family believe he shot her despite there being no evidence. I want to give my cousin some semblance of peace, and the boyfriend too as my cousin and her family harass him in public if they happen to be in the same place because they believe he killed her. The police haven't charged him and do not intend to, the case has been ruled a suicide and closed. But they do not believe it. I don't want to risk my friends career by saying what I know but I hate seeing the pain this has caused. Should I tell my cousin what I know? Or just hope the law and courts give them peace?

I just have what the lead detective on the case told me... surely she knows all the facts of the case? My cousin talks about the bruises her friend had (but doesn't know he had them too) or the drug and drink abuse or the cheating etc she just says her friend hated guns and wouldn't of touched it.